Structural impact on the methano bridge in norbornadiene, norbornene and norbornane

https://doi.org/10.1016/j.jpcs.2004.08.018Get rights and content

Abstract

An electronic structural study of the ground electronic states for the chemically similar bicyclic norbornadiene (NBD, C7H8, X1A1), norbornene (NBN, C7H10, X1A′) and norbornane (NBA, C7H12, X1A1) molecules is provided quantum mechanically. Initially, the unique orbital imaging capability of electron momentum spectroscopy is used to validate which of the quantum mechanical models available to us for these calculations best represents these species. Thereafter, individual molecular point group symmetry is incorporated in the calculations with energy minimization in the search for equilibrium geometries of the species using MP2/TZVP and B3LYP/TZVP models. The optimized geometries compare favourably with available crystallographic results and also build confidence in cases where the crystallographic results are ambiguous. The present study aims to reveal the particular subtle structural deviation of the species, which results in significant molecular property differences among these organic compounds. This work intends to probe bonding information of the species and the impact, on the seven member carbon skeleton, as the C6-point double bondC double bonds of NBD are progressively saturated by hydrogen atoms to give NBN and NBA. Significant changes observed through the present work include: (i) the seven member carbon skeleton tends to relax the strain whenever possible and (ii) the ethano ring experiences greater structural changes than the methano bridge. The methano bridge (C(1)–C(7)–C(4)) of the less symmetric NBN molecule (Cs) tilts to the single C–C bond side of the ethano ring of the molecule (rather than the C6-point double bondC side), producing a dihedral angle of 8.7° between plane H–C(1)–C(4) (the yz-plane) and plane C(1)–C(7)–C(4). Our work suggests that it is this unique dihedral angle in NBN which causes the molecules exo-reactivity and is also responsible for the extra activity of its C6-point double bondC bond.

Introduction

The chemically similar, highly strained, bicyclic hydrocarbons norbornadiene (C7H8, NBD), norbornene (C7H10, NBN) and norbornane (C7H12, NBA) comprise one of the key groups of compounds in structural and synthetic chemistry. The rigid bicycle carbon skeleton of the three species has often been employed to fix geometric variables in structure/reactivity studies, as well as in the investigation of the relationship between structural and spectroscopic properties [1]. The six-membered (ethano) ring is held in a boat conformation and thus serves as a model for the transition state for chair–chair inter-conversion in synthetic chemically important six-membered rings [1]. Moreover, the bridging C(7) atom subtends a less than optimum angle (109.47°) for a saturated linkage and hence is expected to be a major contributor to strain effects. Owing to their globular nature and the absence of strong, directional intermolecular forces, the three species are orientationally disordered, and exhibit plastic phases at ambient temperature and pressure [2]. However, other molecular properties such as chemical reactivity differ significantly between these species. A detailed knowledge of their electronic structures could reveal how molecular bonding information responds to their substantial strains and how their significantly different properties could reflect their subtle structural differences.

It is well known that the molecular properties of species depend critically on their detailed electronic structures. The three species studied here are associated in a close relationship: point group C2v for both unsaturated NBD and saturated NBA and point group Cs for NBN. Norbornane (C2v) is a key compound in structural chemistry and it demonstrates challenges to crystallographic structural determinations. Its derivatives have held a prominent position in the investigation of phenomena associated with non-classical ions [3], such properties of bicyclic compounds wherein they differ from the corresponding acyclic alkane are rationalized in terms of several types of ‘strain’ imposed by their geometries. Bicyclic molecules are also of interest because, compared to acyclic species, the relatively rigid structures of the carbon skeleton lead to unambiguous orientations and magnitudes of separation of substitutions. The absence of complications from considering averages over several widely different conformations has additionally made this group of molecular species attractive for testing numerous models [4].

A C6-point double bondC double bond is usually planar: the two carbon atoms with sp2 hybridization, and the four atoms attached to them lie in a common plane. However, if the double bonds are incorporated into strained bicyclic systems, such as NBN and NBD, considerable deviations from planarity of double bonds can occur. Hence if the bond angles deviate from those idealized bond angles of 109.47 and 120.0° for hybridized carbons, the molecules become strained. The seven-member unsaturated species (NBN and NBD) do not have a plane of symmetry passing through the sp2 carbons so that their double carbon bonds are associated with non-planarity and strain. The experimental exo-reactivity related to the double bond of the NBN molecule has puzzled chemists for many years [5]. Norbornene is considered as one of the prototype strained olefins, due to its extreme chemical reactivity, undergoing elimination, dimerisation, addition and isomerisation [6], as well as exo-selectivity [7]. Norbornadiene (NBD), on the other hand, is an unconjugated, strained and non-planar diene. It is a prototype for through space and through bond interaction studies, with our recent electron momentum spectroscopy (EMS) study confirming the through space interaction dominance in NBD [8], [9].

There are many possible quantum mechanical (QM) models that we could employ in the electronic structure calculations of this paper. It would therefore be nice to have an a priori indication of which of these QM models might provide a reliable representation of the molecules in question. The unique orbital imaging capability of EMS [8], [9] provides us with a mechanism for achieving precisely this. In this technique EMS measures orbital momentum densities (MDs) for the molecular orbitals of the species in question, and these are then compared in detail to those correspondingly calculated using the available basis states (from e.g. GAUSSIAN, GAMESS or DGAUSS). On the basis of this rather detailed comparison optimum models for the species of interest can thus be determined and then used in our detailed electronic structural calculations.

Section snippets

Experimental and theoretical EMS details

The 18 molecular orbitals (MOs) of the complete valence region of NBD, 19 MOs of NBN and 20 MOs of NBA were, respectively, investigated in a series of experimental runs using the Flinders symmetric non-coplanar EMS spectrometer. Details of this coincidence spectrometer and the method of taking the data can be found in Brunger and Adcock [10] and Weigold and McCarthy [11] and so we do not repeat them again here.

The high purity (>99.5%) sample (NBD, NBN or NBA) is admitted into the target chamber

Comparison between experimental and theoretical momentum distributions

The comparisons of calculated orbital momentum densities with experiment may be viewed as an exceptionally detailed test of the quality of the basis set. The results from this process for some illustrative examples in NBD, NBN and NBA are now presented.

Our EMS results for NBD have been discussed in detail previously [8], [9]. Here, we simply reproduce and discuss our experimental and theoretical orbital momentum densities for the 3b2 orbital of norbornadiene. In Fig. 1 we see that the shape of

Challenges to crystallographic experiments and existing theoretical studies of the species

Experimental crystallography measurements on the structural determination of these species are challenging tasks. It is known that electron diffraction (ED) and powder X-ray diffraction (XD) experiments serve as important molecular structural measurement means. In these experiments, even for most molecules of interest, it is not possible to determine the molecular structures completely, accurately and unambiguously from the intensities obtained from the ED or XD experiments alone [19]. For

Molecule orientation in space and computational details

Molecule orientation and symmetry are an essential part of molecular spectroscopy. The presence or absence of symmetry has consequences on the appearance of spectra, the relative reactivity of groups, and many other aspects of chemistry including the way that molecular orbitals and interactions are presented [28]. The bicyclic seven-carbon skeleton is orientated in a Cartesian coordinate system and the carbon atom numbering scheme is given in Fig. 4. The seven-carbon skeleton numbering starts

Molecular electronic structural analysis for NBD

Norbornadiene (NBD, C7H8) in its ground electronic state (X1A1) is a closed shell molecule with 25 doubly occupied molecular orbitals (MOs), including 7 core MOs and 18 valence MOs. This molecule is very symmetric with two non-conjugated C6-point double bondC bonds and a C2v point group symmetry. In order to reveal the structural details, Fig. 5 provides three two-dimensional (2D) views of the optimized geometry of NBD with (i) being the birds-eye view of the xy-plane and (ii) and (iii) being side views of the

Methano ring in the carbon skeleton of NBN

Norbornene (C7H10) exhibits very different properties, such as its chemical reactivity and exo-selectivity. Its electronic structure must be responsible for these unique properties. Due to its Cs point group symmetry, the electronic ground state is X1A′, which has 26 doubly occupied MOs, including 7 core MOs and 19 valence MOs. The optimized molecular geometry of the molecule is reported in Table 2, together with results from other theoretical and experimental work. Fig. 6(i)–(iii) give 2D

Saturated bicyclic hydrocarbon of NBA

The ground electronic state for fully saturated NBA is X1A1, and its 27 doubly occupied MOs consist of 7 core MOs and 20 valence MOs. A summary of our electronic structural calculations for the NBA ground state is given in Table 3. All the C–C bonds are strained single bonds in NBA. However, due to their different bonding mechanisms the respective bond lengths are quite different: the hydrogen saturated C(2)–C(3) bond becomes the longest C–C bond while the bridge C(1)–C(7) bond is the shortest

An electronic structural comparison of NBD, NBN and NBA

Norbornadiene, norbornene and norbornane have substantial electronic structures. If these three molecules orientate in the same Cartesian coordinate system as defined in Section 5, the correlation diagram for their irreducible representations is given by a1, b1→a′ and a2, b2→a″, when one of the C6-point double bondC double bonds of NBD is saturated by H atoms or when one of the ethano single bonds of NBA becomes a C6-point double bondC double bond. In both scenarios the resulting structure becomes that of the NBN, a species with

Conclusions

We have reported on some of our results from our EMS studies into the respective valence electronic structures of NBD [8], [9], NBN and NBA. On the basis of a detailed comparison between our calculated and measured orbital momentum densities, for each molecule, we found that the optimum (but not perfect) representation of said species was provided by the TZVP basis set. This result was then used in our detailed structural calculations that followed.

Highly accurate, unambiguous and conclusive ab

Acknowledgements

The authors acknowledge the Australian Partnership for Advanced Computing (APAC) for using the Compaq SC Alphaserver Cluster National Facilities. One of the authors (FW) would like to acknowledge useful discussions with Dr Harry Quiney, while we all would like to thank Ms Kate Nixon for her assistance on the literature survey. Finally, MJB thanks the organising committee of Sagamore XIV for supporting his attendance at that meeting.

References (49)

  • J. Spanget-Larsen et al.

    Tetrahedron Lett.

    (1982)
  • A.J.G. Barwise et al.

    Chem. Phys. Lett.

    (1976)
  • H. Mackenzie-Ross et al.

    J. Elec. Spectrosc. Relat. Phenom.

    (2002)
  • R.R. Sauers

    Tetrahedron

    (1998)
  • G. Wipff et al.

    Tetrahedron Lett.

    (1980)
  • A. Choplin

    Chem. Phys. Lett.

    (1980)
  • C.R. Castro et al.

    J. Am. Chem. Soc.

    (1968)
  • M. Brunelli et al.

    Z. Kristallogr.

    (2001)
  • T.P. Nevell et al.

    J. Phys. Soc.

    (1939)
  • J.F. Chiang et al.

    J. Mol. Struct. (Theochem)

    (1987)
  • N. Koga et al.

    J. Phys. Org. Chem.

    (1990)
  • H. Mackenzie-Ross et al.

    J. Phys. Chem. A

    (2002)
  • M.J. Brunger et al.

    J. Chem. Soc., Perkin Trans.

    (2002)
  • E. Weigold et al.

    Electron Momentum Spectroscopy

    (1999)
  • G. Bieri et al.

    Helv. Chim. Acta

    (1977)
  • P. Bischof et al.

    Helv. Chim. Acta

    (1969)
  • M. Getzlaff et al.

    J. Elec. Spectrosc. Relat. Phenom.

    (1988)
  • P.R. Bevington et al.

    Data Reduction and Error Analysis for the Physical Sciences

    (1990)
  • W. Kohn et al.

    Phys. Rev. A

    (1965)
  • M.T. Michalewicz et al.
  • I.E. McCarthy et al.

    Rep. Prog. Phys.

    (1991)
  • N.L. Allinger et al.

    J. Am. Chem. Soc.

    (1989)
  • L. Doms et al.

    J. Am. Chem. Soc.

    (1983)
  • M.J. Shephard et al.

    J. Phys. Chem.

    (1995)
  • Cited by (0)

    View full text